FIG. 22 is a sectional view of sensor device 600 disclosed in Patent Literature 1 and to be used for detecting, e.g. viruses. Sensor device 600 includes prism 601, metal layer 602 disposed on a lower surface of prism 601 and having a flat surface, insulating layer 603 disposed on a lower surface of metal layer 602 and having a flat surface and a predetermined dielectric constant, and acceptor 604 fixed to a lower surface of insulating layer 603.
Surface plasmon wave (i.e. compression wave of electrons) exists at an interface between metal layer 602 and insulating layer 603. Light source 605 is disposed above prism 601 and supplies P-polarized incident light to prism 601 under a condition of total reflection. This incident light causes an evanescent wave on surfaces of metal layer 602 and insulating layer 603. The light totally reflected on metal layer 602 is received by detector 606 to detect an intensity of the light.
When a wave-number matching condition in which a wave number of the evanescent wave matches with a wave number of the surface plasmon wave is satisfied, the light energy supplied from light source 605 is used for exciting the surface plasmon wave, so that the intensity of the reflected light decreases. The wave-number matching condition depends on an incident angle of the light supplied from light source 605. Therefore, while the incident angle is changed, an intensity of reflected light is detected with detector 606. The intensity of the reflected light decreases at a certain incident angle.
A resonant angle at which the intensity of the reflected light takes a minimum value depends on the dielectric constant of insulating layer 603. When a specific bound substance is formed on the lower surface of insulating layer 603, the dielectric constant of layer 603 changes, and the resonant angle changes accordingly. This specific bound substance is produced by acceptor 604 and an analyte (i.e. a target to be measured in a sample) that are specifically bound together. By monitoring the change of the resonant angle, a binding strength of the specific bound substance or a speed of the binding can be detected.